1. Field of the Invention
This invention relates to a process for drawing gel-spun polyethylene multi-filament yarns and to the drawn yarns produced thereby. The drawn yarns are useful in impact absorption and ballistic resistance for body armor, helmets, breast plates, helicopter seats, spall shields, and other applications; composite sports equipment such as kayaks, canoes, bicycles and boats; and in fishing line, sails, ropes, sutures and fabrics.
2. Description of the Related Art
To place the invention in perspective, it should be recalled that polyethylene had been an article of commerce for about forty years prior to the first gel-spinning process in 1979. Prior to that time, polyethylene was regarded as a low strength, low stiffness material. It had been recognized theoretically that a straight polyethylene molecule had the potential to be very strong because of the intrinsically high carbon—carbon bond strength. However, all then-known processes for spinning polyethylene fibers gave rise to “folded chain” molecular structures (lamellae) that inefficiently transmitted the load through the fiber and caused the fiber to be weak.
“Gel-spun” polyethylene fibers are prepared by spinning a solution of ultra-high molecular weight polyethylene (UHMWPE), cooling the solution filaments to a gel state, then removing the spinning solvent. One or more of the solution filaments, the gel filaments and the solvent-free filaments are drawn to a highly oriented state. The gel-spinning process discourages the formation of folded chain lamellae and favors formation of “extended chain” structures that more efficiently transmit tensile loads.
The first description of the preparation and drawing of UHMWPE filaments in the gel state was by P. Smith, P. J. Lemstra, B. Kalb and A. J. Pennings, Poly. Bull., 1, 731 (1979). Single filaments were spun from 2 wt. % solution in decalin, cooled to a gel state and then stretched while evaporating the decalin in a hot air oven at 100 to 140° C.
More recent processes (see, e.g., U.S. Pat. Nos. 4,551,296, 4,663,101, and 6,448,659) describe drawing all three of the solution filaments, the gel filaments and the solvent-free filaments. A process for drawing high molecular weight polyethylene fibers is described in U.S. Pat. No. 5,741,451. The disclosures of these patents are hereby incorporated by reference to the extent not incompatible herewith.
Although gel-spinning processes tend to produce fibers that are free of lamellae with folded chain surfaces, nevertheless the molecules in gel-spun UHMWPE fibers are not free of gauche sequences as can be demonstrated by infra-red and Raman spectrographic methods. The gauche sequences are kinks in the zig-zag polyethylene molecule that create dislocations in the orthorhombic crystal structure. The strength of an ideal extended chain polyethylene fiber with all trans —(CH2)n—sequences has been variously calculated to be much higher than has presently been achieved. While fiber strength and multi-filament yarn strength are dependent on a multiplicity of factors, a more perfect polyethylene fiber structure, consisting of molecules having longer runs of straight chain all trans sequences, is expected to exhibit superior performance in a number of applications such as ballistic protection materials.
A need exists for gel-spun multi-filament UHMWPE yarns having increased perfection of molecular structure. One measure of such perfection is longer runs of straight chain all trans —(CH2)n— sequences as can be determined by Raman spectroscopy. Another measure is a greater “Parameter of Intrachain Cooperativity of the Melting Process” as can be determined by differential scanning calorimetry (DSC). Yet another measure is the existence of two orthorhombic crystalline components as can be determined by x-ray diffraction. It is among the objectives of this invention to provide methods to produce such yarns by drawing, and the yarns so produced.